Screening strategy for developing thermoelectric interface materials.

Thermoelectric interface materials (TEiMs) are essential to the development of thermoelectric generators. Common TEiMs use pure metals or binary alloys but have performance stability issues. Conventional selection of TEiMs generally relies on trial-and-error experimentation. We developed a TEiM screening strategy that is based on phase diagram predictions by density functional theory calculations. By combining the phase diagram with electrical resistivity and melting points of potential reaction products, we discovered that the semimetal MgCuSb is a reliable TEiM for high-performance MgAgSb. The MgCuSb/MgAgSb junction exhibits low interfacial contact resistivity (ρc <1 microhm square centimeter) even after annealing at 553 kelvin for 16 days. The fabricated two-pair MgAgSb/Mg3.2Bi1.5Sb0.5 module demonstrated a high conversion efficiency of 9.25% under a 300 kelvin temperature gradient. We performed an international round-robin testing of module performance to confirm the measurement reliability. The strategy can be applied to other thermoelectric materials, filling a vital gap in the development of thermoelectric modules.

Geometrical Optimization and Thermal-Stability Characterization of Te-Free Thermoelectric Modules Based on MgAgSb/Mg3 (Bi,Sb)2.

Solid-state thermoelectric (TE) technology is a promising approach to harvest low-grade waste heat (<573 K) and converts it to useful electricity in industrial and civilian settings. After decades of efforts in improving the figure-of-merit (zT) of TE materials, the development of advanced modules has started springing up in recent years. Although high-performance modules have been largely reported based on the successful material improvement, it remains less investigated how and whether the module-level designs can further increase the conversion efficiency. Herein, following the recent demonstration of a tellurium (Te)-free TE generator, an increase is demonstrated in the efficiency by reducing both the electrical and thermal energy losses through simply optimizing geometric factors of filling factor and leg-pair numbers. These module-level optimizations enable a record conversion efficiency of 8.2% under a ∆T ≈ 260 K, thus fulfilling 90% of the theoretical efficiency of the materials and solidly exceeding the Bi2 Te3 modules. Furthermore, module robustness against > 10 160 thermal cycles while preserving a relative efficiency of 95% is demonstrated. These findings highlight the importance of the optimization strategy at the module level and demonstrate the feasibility of using Te-free thermoelectric compounds to harvest the omnipresent low-grade heat.

Reduced Lattice Thermal Conductivity for Half-Heusler ZrNiSn through Cryogenic Mechanical Alloying.

The ZrNiSn-based half-Heusler compounds are promising for thermoelectric applications in the mid-to-high temperature range. However, their thermoelectric performance was greatly limited due to the remaining high thermal conductivity, especially the lattice thermal conductivity. Herein, we report the synthesis of pristine half-Heusler ZrNiSn through direct mechanical alloying at a liquid nitrogen temperature (i.e., cryomilling) followed by spark plasma sintering. It is shown that the onset sintering temperature is greatly reduced for the cryomilled powders with a high density. A reduced thermal conductivity is subsequently realized from room temperature to 700 °C in the cryomilled samples than the one that was differently prepared (from 7.3 to 4.5 W/m K at room temperature). The pronounced reduction in thermal conductivity of ZrNiSn yields a maximum zT of ∼0.65 at 700 °C. Our study shows the possibility of cryomilling in advancing the thermoelectric performance through enhanced phonon scattering.

High-Pressure-Sintering-Induced Microstructural Engineering for an Ultimate Phonon Scattering of Thermoelectric Half-Heusler Compounds.

Thermal management is of vital importance in various modern technologies such as portable electronics, photovoltaics, and thermoelectric devices. Impeding phonon transport remains one of the most challenging tasks for improving the thermoelectric performance of certain materials such as half-Heusler compounds. Herein, a significant reduction of lattice thermal conductivity (κL ) is achieved by applying a pressure of ≈1 GPa to sinter a broad range of half-Heusler compounds. Contrasting with the common sintering pressure of less than 100 MPa, the gigapascal-level pressure enables densification at a lower temperature, thus greatly modifying the structural characteristics for an intensified phonon scattering. A maximum κL reduction of ≈83% is realized for HfCoSb from 14 to 2.5 W m-1 K-1 at 300 K with more than 95% relative density. The realized low κL originates from a remarkable grain-size refinement to below 100 nm together with the abundant in-grain defects, as determined by microscopy investigations. This work uncovers the phonon transport properties of half-Heusler compounds under unconventional microstructures, thus showing the potential of high-pressure compaction in advancing the performance of thermoelectric materials.

Towards tellurium-free thermoelectric modules for power generation from low-grade heat.

Thermoelectric technology converts heat into electricity directly and is a promising source of clean electricity. Commercial thermoelectric modules have relied on Bi2Te3-based compounds because of their unparalleled thermoelectric properties at temperatures associated with low-grade heat (<550 K). However, the scarcity of elemental Te greatly limits the applicability of such modules. Here we report the performance of thermoelectric modules assembled from Bi2Te3-substitute compounds, including p-type MgAgSb and n-type Mg3(Sb,Bi)2, by using a simple, versatile, and thus scalable processing routine. For a temperature difference of ~250 K, whereas a single-stage module displayed a conversion efficiency of ~6.5%, a module using segmented n-type legs displayed a record efficiency of ~7.0% that is comparable to the state-of-the-art Bi2Te3-based thermoelectric modules. Our work demonstrates the feasibility and scalability of high-performance thermoelectric modules based on sustainable elements for recovering low-grade heat.

Anisotropic thermoelectric properties of layered compound SnSe2.

Similar to high performance SnSe thermoelectrics, SnSe2 is also a layered structured semiconductor. However, its anisotropic thermoelectric properties are less experimentally investigated. In this work, Cl-doped SnSe2 bulk materials are successfully prepared, and their thermal stability and anisotropic transport properties are systematically studied. Unexpectedly, different from the theoretical prediction and other typical layered thermoelectric compounds like Bi2Te3, the out-of-plane zTc value is higher than in-plane zTa for the same composition. The zT value is significantly enhanced by Cl doping. A maximum zTc of ∼0.4 at 673 K is achieved in SnSe1.88Cl0.12, twice higher than previously reported Cl-doped SnSe2 synthesized by the solvothermal method.